1. Field of the Invention
The invention relates to integrated circuits and, more specifically, it relates to the making of irreversible locking elements that can be used to permanently modify the functions of certain parts of the circuit or access to these parts.
The standard term "fuse" shall be used hereinafter to designate these locking elements, it being understood nevertheless that this is a figure of speech referring to a function and that the structure of these elements does not necessarily implement a physical fusing operation.
2. Discussion of the Related Art
A common application of fuses in the field of integrated circuits relates to chip cards: it is often sought to prohibit access to memory zones or to certain functions, the prohibition being applied progressively as these cards go through the stages of being manufactured and marketed.
A typical example is the following one: the card manufactured by a card manufacturer must initially have all its points of access authorized and all its functions authorized if only for testing purposes. Then, certain confidential data elements have to be made inaccessible: for example, the card manufacturer does not wish the programs of the operating system (which are in the read-only memory in the chip) to be accessible in read mode. One or more fuses are blown to prohibit this hitherto possible reading operation. The card may then be delivered, for example to a provider of a service (a bank, telephone company, etc.). The provider of the service will place other programs and confidential data in the card (in a programmable memory), and may wish to physically prohibit read and write access to these programs and data elements. Other fuses could be blown at this point in time.
Fuses presently used in this type of application include, for example:
polycrystalline silicon fuses in which a breakdown current is applied to a very thin layer of polycrystalline silicon, which gets heated and evaporates, opening the circuit in which it is placed. This type of fuse has problems of reliability related to poor evaporation and the risk that the conductor will be reconstituted over a period of time.
oxide breakdown EEPROM type electrically programmable memory cellsin which the blank state of the cell is likened to an intact state of the fuse and the programmed state is likened to a blown state. This type of lock is very reliable if an external programming voltage of sufficient energy is used, but it is far less reliable if the voltage used is a voltage internal to the integrated circuit chip. The use of an external voltage has several drawbacks. For example, greater cost due to an additional supply terminal, which is also troublesome for the user who has to plan for this voltage. Furthermore, the breakdown occurs through the application of a series of voltage pulses, but if the breakdown is not successful at the first attempt it becomes pregressively more difficult;
blank EEPROM memory cells with a zener diode on source or drain, which can be erased once but never again reprogrammed. A high voltage is necessary for breakdown with the conventional drawbacks that this entails. The drawbacks include the need for an external voltage with a specific access pad or an internal voltage, but an internal voltage is less reliable and requires providing a voltage pull-up circuit on the chip.
One general aim of the invention is to provide a particularly simple type of fuse that can work at very low voltage. Other aims of the invention will become apparent upon reading the following description.